A great number of portable audio devices such as cellular telephones, MP3 players, CD players, XM radio receivers and other devices are not readily useable with the audio systems found in most automobiles. In addition, some other aftermarket devices, such as global positioning satellite (GPS) navigation systems, produce audio prompts that are not compatible with current automobile audio systems. Most automobile audio systems do not provide auxiliary audio inputs for external devices such as the above. Unlike home stereo systems which typically have several external input options and one or more controls for selecting one of those input options, automotive audio systems typically have few or none. Automotive audio systems have limited available space for selection controls and input connectors on the front panel and similarly limited space for input connectors on the back of the system.
Audio output from some external audio devices, specifically from MP3 players and XM radio receivers, can be inputted to a car radio receiver by first using the audio signal from the external device to frequency modulate a radio frequency signal having the same frequency as that of the IF frequency of the receiver. Then a low power FM transmitter is used to transmit the frequency modulated signal directly to the IF amplifier in the receiver. Following which the receiver demodulates the received signal, thereby recovering the audio signal.
In another technique, audio output from external audio devices can be inputted to the car radio receiver by first using the audio signal from the external device to frequency modulate a radio frequency signal having a frequency that lies within the reception band of the receiver. Then a low power FM transmitter transmits the frequency modulated signal directly to the antenna of the receiver. The audio signal is then extracted by the receiver just as if it were received from a commercial FM radio station. For this technique, the FM radio must be tuned to the selected transmission frequency.
Also, for those car stereos which have a tape player, devices having the same shape as a tape cassette are available which include a transducer for magnetically coupling the audio signal from the external audio source to the magnetic tape head in the tape player.
In addition, a few automotive audio systems currently do have auxiliary inputs for MP3 players or GPS navigation systems. But, such systems are primarily limited to specific brand combinations of specific devices.